I was using Wireshark a few hours ago, and started a capture. A couple of hours later, it closed unexpectedly. I was not able to save the capture that I started, but I really need the VOIP call that I recorded earlier. Will I be able to recover it?

You are very probably out of luck. The temporary file containing the capture data is probably not present any more. As mentioned by @JeffMorriss, the temporary file is probably still present, and you should be able to use it (althouhg you should probably start by copying it somewhere it will be safe to work with) by opeining it with Wireshark.
For future reference, if Wireshark was capturing for the entire time, you probably ran into the known issue with Wireshark running out of memory. You can avoid this issue by using dumpcap directly for long-running captures, and then processing it post-mortem using Wireshark, possibly after reducing the file size by splitting the capture using editcap.